Electrostatic coating methods and apparatus

ABSTRACT

An apparatus for coating a surface of an article with particles of a powder. A gas-permeable plate is mounted in a chamber and is used to support a layer of particles in the chamber. Means passes gas through the plate to cause the particles to form a cloud in the chamber. The article to be coated is in the cloud of particles. An electrode means is above the article to be coated with particles. Means establishes on electrostatic field between the electrode means and the article for charging the particles and collecting their deposition on the article.

United States Patent Indianapolis, Ind.

ELECTROSTATIC COATING METHODS AND APPARATUS 10 Claims, 4 Drawing Figs.

US. Cl 118/630, 1l7/33.5,117/17,118/400.5,118/D1G.5

Int. Cl B051) 5/02 Field of Search 118/621- Primary ExaminerPeterFeldman Attorneys-Merrill N. Johnson, Harry E. Downer, Jerry E.

l-lyland, David H. Badger and Verne A, Trask ABSTRACT: An apparatus forcoating a surface of an article with particles of a powder. Agas-permeable plate is mounted in a chamber and is used to support alayer of particles in the chamber. Means passes gas through the plate tocause the particles to form a cloud in the chamber. The article to becoated is in the cloud of particles. An electrode means is above thearticle to be coated with particles. Means establishes on electrostaticfield between the electrode means and the article for charging theparticles and collecting their deposition on the article.

PATENTEflJuLzoen 3.593678 sum 2 [1F 2 II IIIIIIIIIIIII'II 3| 32 o I I0F/ 9.2

[I'll/[111111111] F/g. 4 INVENTOR EMERY- P. MILLER ELECTROSTATIC COATINGMETHODS AND APPARATUS This is a division of application Ser. No.544,594, filed Apr. 22, l966,now U.S. Pat. No. 3,513,01 I.

This invention is concerned with apparatus for coating televisionpicture tubes with phosphors with the aid of electrostatic forces. It isalso concerned with the application of powders generally to any surfacewhere extreme uniformity of coating is required such as in the formationof electroluminescent plates and the like.

It has heretofore been the practice in the production of picture tubesfor television receivers to apply the phosphor coating to the tube faceeither as dry powders sprayed onto an adherent layer on the glasssurface or as sediments from liquid suspensions which are substantiallyadhered by being sprayed with an overlay of suitable resin. ln the caseof those tubes to be used in color television receivers, the process hasinvolved the application of a binder containing a light sensitizingagent to the inside surface of the tube face, and then the dusting ofthe dry particles of the phosphor thereover to produce a thin adherentpowder layer. The thin powder layer over the sensitized binder layer isthen exposed to a suitable light image of an array of dots, and thematerial within the area of the dots is polymerized onto the tube faceas insoluble resin areas to which the powder is adhered. The remainingmaterial, being soluble, is then removed by a rinse and a grid ofisolated dots in definite array and location is thus formed over theface surface. The process is then repeated again using the sensitizedbinder and a different phosphor. A new array of light dots is projectedonto the surface to promote the polymerization of the material locatedin different areas than the first set of dots. The process is repeatedfor a third time using the sensitized binder and still a third differentphosphor, the dots of the polymerized material being located in stilldifferent areas. There is thus produced a finished coating of thephosphors, preferably one particle deep, over the face surface beingcoated. Since the three specialized groups of dots are each of aphosphor designed to emit, when excited, a different primary color, thecomplete surface is thus adapted to reproduce the entire color spectrum.

Such methods are extremely expensive. The necessity for dusting thephosphors over the tube face, in addition to being wasteful of powder,is time consuming and thus yields low production rates. Further, suchoperations create a high incidence of tube faces which have to berejected by reason of nonuniform coatings. The phosphors employed in thecoating are extremely expensive, and with the high incidence of rejects,substantial quantities of these expensive phosphors are thus wasted.

It is an object of this invention to provide an apparatus for coatingtelevision receiver picture tubes which will overcome the difficultiesand disadvantages discussed above. More specifically, it is an object ofthis invention to provide an apparatus which will rapidly deposit acoating of phosphor powders on television picture tube faces, which willdirect said phosphor powders to and deposit them on a tube face with theaid of electrostatic forces, which will provide a uniform coating on thesurface to be coated, which will produce such surfaces with consistentcharacteristics, and which will be efficient in operation and in the useof powders. A further object of the invention is to provide a method ofcoating such tube faces in which the sensitized binder for adhering thephosphors onto the tube face will serve as an agent for supplyingsurface conductivity to said face for reception of the electrostaticallycharged phosphor particles.

According to one form of the invention for coating color television tubefaces, the uncoated'face has a sensitized binder applied in a uniformlayer to its surface to be coated. The powdered phosphor to be depositedis placed in a fluidized bed and gas, such as air, is passed through itso as to fluidize the material and form a powder cloudabove the bed. Thefluidizing air is turned off and the face inserted into the cloud abovethe powder bed with its surface to be coated facing upward and with thesensitized binder layer in electrical contact with ground. A chargingelectrode is positioned above and spaced from the tube face. Applicationof a potential difference between the electrode and the faceelectrostatically charges the phosphor particles and in an acceleratedmanner directs them onto the sensitized layer as said particles tend tosettle gravitationally from the cloud. Conveniently, during depositionthe electrode and the tube face are moved relative to each other.

After the phosphor particles have been deposited, light is projectedonto the coated surface through a dotted screen to activate thesensitizing agent, polymerize the binder under the illuminated dots, andthus cause the deposited phosphor particles at the dots to be bound ontothe tube surface. After such polymerization, the nonpolymerized binderand phosphor particles at the areas other than the dots are removed fromthe surface by rinsing the surface with a solvent liquid. The sensitizedbinder application, phosphor powder deposition, and polymerization arerepeated twice again, each time using a different phosphor and differentdot locations. This results in a finished coating consisting of a triplearray of nonoverlapping dots uniformly disposed over the surface to thetube; each dot having uniform thickness of phosphor.

Other objects and features of the invention will become apparent fromthe more detailed description which follows and from the accompanyingdrawings in which:

FIG. 1 is a vertical section through an apparatus embodying theinvention with the tube face in place to be coated;

FIG. 2 is a partial section through the apparatus with the tube facewithdrawn from the coating chamber;

FIG. 3 is a plan view of the assembly taken through A-A in FIG. 1; and

FIG. 4 is an in view of the carriage mounting arrangement.

In the production of picture tubes for color television receivers, acoating of phosphors is applied to the tubes. Said coating, which issubstantially only one particle thick, is formed of phosphors having aparticle size of about 3 microns which are adapted to produce a visiblered, blue, and green phosphor escence when bombarded by electrons.

To produce such a coating, we clean the tube face and apply thereto, asby spraying, brushing, slushing, or the like, a polar binder solutioncontaining a water soluble light-activatable sensitizing agent.Desirably, said solution comprises about 3 percent of polyvinyl alcoholand about one-half percent ofa chromic salt, such as ammoniumdichromate, dissolved in isopropyl alcohol and water in a lzl ratio. Thepolyvinyl alcohol serves as a binding agent for the phosphors to bedeposited. The chromic salt, in addition to providing light sensitivity,acts in combination with the polar solvents, isopropyl alcohol andwater, to increase the conductivity of the solution while in the wet orpartially dried state to thereby given the surface to be coated asurface resistivity in the range of from about 10' to about 10 ohms persquare. After the solution has been uniformly applied to the insidesurface of the tube face, it is allowed to dry to a tacky condition. Thetube face is then ready for the first phosphor application. It is placedin the coating apparatus illustrated in FIG. 1 in the position shown inFIG. 2.

As shown, the coating apparatus comprises a chamber ll formed from fourinterconnecting sidewalls 12 of electrically insulating materialprojecting upwardly from a bottom wall 13. Conveniently, an outer jacket14 formed from electrically insulating material and supported on aplurality of feet 15 extends around the chamber ll. A pair of rods 16extend across the jacket 14 to support the chamber 11 in spaced relationthereto whereby said jacket catches any of the phosphor powder thatflows over the upper edges of the chamber. As shown, the upper end ofthe jacket 14 is spaced slightly above the upper ends of the chamberwalls 12, and the lower end 17 of said jacket has a frustoconicalconfiguration for collecting the powders in the jacket. Conveniently, afan 18 is mounted at the bottom of the jacket for conveying thenondeposited powder from the jacket to a collector reservoir 19.

An arm also of insulating material projects upwardly from the jacket 14and extends transversely across the chamber 11. Adjustably affixed tosaid arm is a rod of conducting material 21 to which is attached anair-driving motor 22. Attached to the rotatable shaft of air motor 22 isa charging electrode system 23 composed of a plurality of pointsextending outwardly of a plane frame member. The electrode is of thesame planar shape as the tube face to be coated. In the modificationshown this is essentially square. The rod 21 is connected electricallyby way of fitting 24 and wire 25 to a high-voltage supply 26 whose otherend is grounded as at 27. Thus, upon actuation of the motor 22 andenergization of the high-voltage supply 26, the electrode 23 will beelectrically charged with respect to ground and will be rotated withrespect to the chamber 11.

One of the walls 12 of chamber 11 has an opening 28 in it located abovethe level of the fluidized powder bed which is large enough and soshaped to admit the tube face I0 when it is held in a horizontalposition. A similar matching opening 29 of slightly larger dimension islocated in the corresponding surrounding wall 14.

Attached to the side of the enclosure having openings 28 and 29, thereis a bracket member 30 which forms a support frame. Attached to the sidemembers of this frame there are two sets of rollers 31 and two sets ofrollers 32. These rolls are arranged to carry the tube face supportcarriage 33. Rolls at 31 are arranged above and below the angle sidebars of carriage 33 as shown in FIG. 4. Rolls at 32 are single rollsarranged to support the angle only from below in the manner of the lowerroll of rollers 31. The carriage thus can be moved from the positionshown in FIG. 2 to that shown in FIG. 1 without tipping.

The carriage carries at its one end a cross bar 35 and at its other enda flexible thin metal plate 36. Appropriately placed along the carriageis another similar plate 37. Plates 36 and 37 are sized so that theywill completely close the opening 28 from opposite sides of the panel 14when the carriage is all the way toward the right as in FIG. 1 or allthe way to the left as in FIG. 2. A third flexible plate 38 is attachedto the carriage 33 at a position such that it will close opening 29 inwall I4 when the carriage is all the way toward the right. It istherefore made slightly larger than opening 29.

An air permeable plate 41 is mounted on the chamber II above the bottomwall 13 and supports the phosphor particles 42 to be deposited. Air isintroduced into the compartment 43 between the bottom wall 16 and theplate 41 through an air inlet 44 connected to a source of pressurizedair 45. The air moves upwardly from the compartment 43 through the plate41 to maintain the phosphor particles 42 in suspension immediately abovesaid plate and thus provide a fluidized bed of said particles. Byincreasing the flow rate at the inlet 44, the particles will billowupwardly from the bed in the chamber and thus form a cloudhaving aparticle density substantially less than the bed extending from the bedto the top opening of enclosure 11; the degree of billowing beingdependent upon the flow rate at the inlet 44. The cloud will essentiallypour out over the sidewalls l2 and settle into the space between 12 and14 to be recovered by fan 18 and collector 19.

With the carriage all the way to the left, the tube face to which thealcohol solution has been applied, is positioned on carriage 33 as shownin FIG. 2. A spring clip 39 is attached to the carriage and arranged sothat when the face is slipped and provides an average voltage gradientbetween the points on electrode 23 and the tube face of at least 5kilovolts per inch. With the coating on the tube face being grounded andthe electrode being charged, the phosphsr particles in the cloud fromthe bed which are in the field extending between the electrode and tubeface acquire an electrostatic charge and are thus attracted to anddeposited onto the tube face. With the electrode rotating, a uniformvoltage gradient will be established between the head and' tube face toinsure a uniform deposition of the phosphor particles onto the tubeface.

Phosphor particles which were generated in the cloud before theintroduction of the face will flow over into the space between thechamber 11 and jacket 14 and can be reclaimed in the reservoir 45.

An example of the deposition step of our invention may be described asfollows: The inside of an 1 l-inch tube face was cleaned and then washedwith the sensitized binder, after which it was allowed to dry for 6minutes at a relative humidity of 40 percent and a room temperature ofF. It was then in a tacky condition. The face was then placed on thecarriage 33 outside of the chamber 11, which chamber had a length andwidth of 12 inches and an overall height of IS inches. The plate 42 waslocated 5 inches above the chamber bottom wall I6. The pins of electrodehead 23 were 1 inch above the top of chamber ll. When the face was movedinto the chamber, it was 4 inches above the porous plate 42. Thisprovided a 12- inch spacing between the electrode and tube face. Theelectrode was rotated at r.p.m., and air was introduced into thecompartment 43 at the rate of 5 c.f.m. When the phosphor particles hadbillowed to fill the chamber 11 and the face had been introduced, theelectrode was charged at 60 kv. This produced a uniform coating over theinside surface ofthe tube face, and said face was removed. If a heaviercoating is desired, the cycle can be repeated. When the desired coatingis obtained, the face can be passed on to further processing.

After the phosphor particles have been deposited, the tube face 10 isremoved from the coating chamber, and a beam of light is projectedthrough a dotted image-producing screen and onto the coated surface. Thelight passing through said screen will polymerize the polyvinyl alcoholto thus retain or affix the phosphor particles as a coating on the tubeface within the dotted illuminated areas. After polymerization, thecoating is rinsed with a suitable solvent, and the nonpolymerizedpolyvinyl alcohol and the phosphor particles not within the dotted areasare rinsed away.

After the dotted coating has dried, a second layer of the bindersolution is applied thereover. The deposition process is repeated in themanner previously described in another fluidizing chamber charged with aphosphor having a different color phosphorescence. After deposition, thepolyvinyl alcohol in the binder solution is polymerized by projecting abeam of light through a second dotted image-producing screen and ontothe coated surface. Said screen has its dotted pattern offset from thedotted pattern of the first screen whereby the second layer of bindersolution will be polymerized in dotted areas adjacent the polymerizeddots of the first application. In this manner, a double array ofnonoverlapping dots of the first and second phosphors will be affixed tothe tube face.

After this second coating has dried, the sequence is again repeatedusing a third layer of binder solution and depositing a third phosphorfrom a third chamber having still a different color phosphorescence.After deposition of the third phosphor, the polyvinyl alcohol in thethird layer of binder solution is polymerized by projecting a beam oflight through a third dotted image-producing screen and onto the coatedsurface. The dot pattern in the third screen is offset from the dotpatterns in the first and second screens whereby the third layer ofbinder solution will be polymerized in dotted areas adjacent thepolymerized dots of the first and second application. In

this manner. a finished coating is produced having a triple array ofnonoverlapping dots of the first, second, and third phosphors affixed tothe tube face.

As explained above, the production of picture tubes for color televisionreceivers requires the use of three different phosphors with theirparticles affixed to the tube face in a triple array of adjacentnonoverlapping dots. In the production of picture tubes for black andwhite television receivers, only one phosphor need be employed, and itdoes not have to be deposited in any dotted pattern on the tube face,but only in a uniform thin coating over said tube face. Therefore, asingle application of the binder solution and the electrostaticdeposition of only one phosphor need be employed in the production ofsuch picture tubes. Further, because such tubes do not require thephosphor to be in a dotted pattern, the entire deposited coating issubjected to light projected onto it directly, and no image-producingscreens need be employed,

Similarily, this same technique can be used to produce a uniform powderlayer on any flat surface by simply placing the plate in the positionoccupied by the tube face. The powders need not be of the type referredto as phosphors, but can be any finely ground powder capable of beingfluidized. The surface need not be nonconducting, but can equally wellbe made of metal. In such case, the adhesive liquid need not supply theelectrical conductivity. Where a uniform layer is required, the bindinglayer need not possess photo chemical properties, but can simply hardenupon drying to bind the particles to the surface.

In case the coating applied by a single application is not adequate, theface or surface can be given further treatment by removing it from thechamber, reestablishing the powder cloud by again turning on and off thefluidizing air, and again inserting the surface into the chamber.

While we have described the electrode as being rotated with respect tothe fixed tube face, it is to be understood, of course, that theelectrode may be fixed. It is also to be understood that deposition maybe obtained without the use of the electric field by following all stepsof the invention with the exception of applying the field. Gravitationalsettling of the powder cloud will depsoit the phosphors on the face butthis occurs at a much slower rate. I

For convenience, reference has been made here in to powder particlessuspended in air. However, said particles may be suspended in gaseousmedium other than air, and air" is intended to include other suitablegases.

lclaim:

1. An apparatus for coating a surface of an article with particles of apowder comprising a deposition chamber, a gaspermeable plate mounted insaid chamber supporting a bed of particles in the chamber, electrodemeans carried above said chamber, means for passing gas through saidplate to cause said particles to move upwardly toward the top of saidchamber to form a cloud of particles suspended in the gas and thenterminating the flow of gas, means for introducing the article into saidchamber and supporting the article below said electrode means and abovethe bed of particles, and means for establishing an electrostatic fieldbetween said electrode means and the article for charging said particlesand accelerating their downward deposition on the article.

2. The invention as set forth in claim 1 with the addition that meansare provided for moving said article and electrode means relative toeach other while maintaining a constant spacing therebetween.

3. The invention as set forth in claim 1 in which said chamber has anopen upper end and a jacket is disposed around said chamber in spacedrelation thereto for collecting any nondeposited particles exiting saidchamber.

4. The invention as set forth in claim 3 with the addition that saidjacket is connected to a collector reservoir and means are provided forconveying the powder in saidjacket to said reservoir.

5. The invention as set forth in claim 1 in which said support means isarranged relative to said chamber in such a way as to provide a loadingposition outside of said chamber and a coating position within saidchamber.

6. An apparatus for coating an article with particles of a powder, comrising a deposition chamber, a gasermeable plate mounte In said chambersupporting a layer 0 the particles in the chamber, an electrode meanscarried above said chamber, an article support means in said chamber,means for forcing gas through said plate to cause said particles to moveupwardly toward the top of said chamber and toward said electrode means,ajacket around said chamber in spaced relation thereto, a support onsaid jacket for supporting said electrode means, and means forestablishing an electrostatic field between said electrode means and anarticle on said support means for charging said particles and effectingtheir deposition on an article on said article support: means.

7. The invention as set forth in claim 6 in which said electrode isconnected to a motor mounted on said electrode support for rotating theelectrode with respect to the article.

8. An apparatus for coating an article with particles of a powder,comprising a deposition chamber having an open upper end, angas-permeable plate mounted in the chamber for supporting a layer ofparticles in said chamber, electrode means carried on said chamber,electrically conductive support means associated with the chamber andadapted to engage and support the article above said layer, means forforcing gas through said plate to cause said particles to move upwardlytoward the top of said chamber, electrode means positioned adjacent theopen upper end of said chamber and above the article to have particlesdeposited thereon, and means for establishing an electrostatic fieldbetween said elec trode means and the article for charging saidparticles and effecting their deposition on the article.

9. The invention as set forth in claim 8 in which ajacket is disposedaround said chamber in spaced relation thereto for collecting anynondeposited particles.

10. An apparatus for coating an article with particles of a powder,comprising a deposition chamber having an open upper end, agas-permeable plate mounted in the chamber for supporting a layer of theparticles in said chamber, electrode means carried in said chamber,hanger means adapted to support the article and to move it from withoutinto said chamber through a suitable opening, means for forcing gasthrough said plate to cause said particles to move upwardly toward theopen top of said chamber, means associated with the movement of saidsupport means for closing said opening when said gas forcing means isactive, a second means associated with the movement of said supportmeans for closing said opening when said gas means is not active, anelectrode positioned near the top of said chamber and above the articleto have particles deposited thereon, and means for establishing anelectrostatic field between said electrode means and article in saidchamber for charging said particles and effecting their deposition onthe article.

2. The invention as set forth in claim 1 with the addition that meansare provided for moving said article and electrode means relative toeach other while maintaining a constant spacing therebetween.
 3. Theinvention as set forth in claim 1 in which said chamber has an openupper end and a jacket is disposed around said chamber in spacedrelation thereto for collecting any nondeposited particles exiting saidchamber.
 4. The invention as set forth in claim 3 with the addition thatsaid jacket is connected to a collector reservoir and means are providedfor conveying the powder in said jacket to said reservoir.
 5. Theinvention as set forth in claim 1 in which said support means isarranged relative to said chamber in such a way as to provide a loadingposition outside of said chamber and a coating position within saidchamber.
 6. An apparatus for coating an article with particles of apowder, comprising a deposition chamber, a gas-permeable plate mountedin said chamber supporting a layer of the particles in the chamber, anelectrode means carried above said chamber, an article support means insaid chamber, means for forcing gas through said plate to cause saidparticles to move upwardly toward the top of said chamber and towardsaid electrode means, a jacket around said chamber in spaced relationthereto, a support on said jacket for supporting said electrode means,and means for establishing an electrostatic field between said electrodemeans and an article on said support means for charging said particlesand effecting their deposition on an article on said article supportmeans.
 7. The invention as set forth in claim 6 in which said electrodeis connected to a motor mounted on said electrode support for rotatingthe electrode with respect to the article.
 8. An apparatus for coatingan article with particles of a powder, comprising a deposition chamberhaving an open upper end, an gas-permeable plate mounted in the chamberfor supporting a layer of particles in said chamber, electrode meanscarried on said chamber, electrically conductive support meansassociated with the chamber and adapted to engage and support thearticle above said layer, means for forcing gas through said plate tocause said particles to move upwardly toward the top of said chamber,electrode means positioned adjacent the open uPper end of said chamberand above the article to have particles deposited thereon, and means forestablishing an electrostatic field between said electrode means and thearticle for charging said particles and effecting their deposition onthe article.
 9. The invention as set forth in claim 8 in which a jacketis disposed around said chamber in spaced relation thereto forcollecting any nondeposited particles.
 10. An apparatus for coating anarticle with particles of a powder, comprising a deposition chamberhaving an open upper end, a gas-permeable plate mounted in the chamberfor supporting a layer of the particles in said chamber, electrode meanscarried in said chamber, hanger means adapted to support the article andto move it from without into said chamber through a suitable opening,means for forcing gas through said plate to cause said particles to moveupwardly toward the open top of said chamber, means associated with themovement of said support means for closing said opening when said gasforcing means is active, a second means associated with the movement ofsaid support means for closing said opening when said gas means is notactive, an electrode positioned near the top of said chamber and abovethe article to have particles deposited thereon, and means forestablishing an electrostatic field between said electrode means andarticle in said chamber for charging said particles and effecting theirdeposition on the article.